1. Field of Invention
The present invention relates to a method for manufacturing a flash memory. More particularly, the present invention relates to a method for manufacturing a two-bit flash memory.
2. Description of Related Art
A conventional flash memory is a type of erasable programmable read-only memory (EPROM), which in turn is a type of non-volatile memory. One of the advantages of flash memory is its capacity for block-by-block memory erasure. Furthermore. the speed of memory erasure is fast, and normally takes just 1 to 2 seconds for the complete removal of a whole block of memory. For most other EPROM, memory erasure can take up to several minutes due to its bit-by-bit operation.
Typically, flash memory is constructed from a metal-oxicle-semiconductor transistor with an electrically changeable threshold voltage. The silicon nitride oxide semiconductor (SNOS) is one kind of flash memory.
FIG. 1 is a schematic, cross-sectional view of a conventional two-bit SNOS memory. The components of the SNOS memory comprise a source/drain region 102 formed in the substrate 100, a thin oxide layer 104 formed on a portion of the substrate 100 between the source/drain regions 102 and a silicon nitride layer 106 formed on the thin oxide layer 104. This structure further comprises a polysilicon gate 108 formed on the silicon nitride layer 106.
When the flash memory stores data, a high voltage is applied to the source/drain region 102 and the polysilicon gate 108 to induce hot electrons. The hot electrons flow from the source/drain region 102 and vertically penetrate through the thin oxide layer 104 near the source/drain region 104 to be trapped in a portion of the nitride layer 106 near the source/drain region 102. Therefore, each end of the silicon nitride layer 106 stores one bit. Specifically, a SNOS memory cell can store two bits.
However, the hot electrons trapped in the silicon nitride layer 106 are unstable at both ends of the silicon nitride layer 106. It is easy to induce a redistribution effect of the hot electrons, so that the hot electrons spread over the silicon nitride layer 106. The hot electrons respectively injected from the source region and the drain region are remixed and spread over the silicon nitride layer 106. Hence, the operation performance of the flash memory is restricted and the problem of over-programming occurs.